drm/amd/display: Know what a pageflip is

[Why]
We were assuming that any commit with allow_modeset == false
was a pageflip.  This was against drm intention and only
worked by sheer luck

[How]
A pageflip is the change from one framebuffer to another

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4683,7 +4683,8 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 		if (!new_crtc_state->active)
 			continue;
 
-		pflip_needed = !state->allow_modeset;
+		pflip_needed = old_plane_state->fb &&
+			old_plane_state->fb != new_plane_state->fb;
 
 		dc_plane = dm_new_plane_state->dc_state;